This project is designed to provide information on the ranges and distributions of the electrophysiological and morphological characteristics of alpha motoneurons and of the interrelated mechanical, histochemical and morphological properties of the muscle fibers innervated by them (i.e., the muscle unit). The motor unit populations in normal animals are compared with those in animals after various conditioning treatments. BIBLIOGRAPHIC REFERENCES: Burke, R.E. and Tsairis, P.: Histochemical and phyysiological profile of a skeletofusimotor beta unit in cat soleus muscle. Brain Res. In press, 1977. Burke, R.E., Strick, P.L., Kanda, K., Kim, C.C. and Walmsley, B.: Anatomy of the medial gastrocnemius and soleus motor nuclei in the cat spinal cord. J. Neurophysiol. 40: 667-680, 1977.